The term "Rogowski coil", sometimes also known as a "Rogowski torus", conventionally designates a conductor winding coiled on a toroidal former made of non-ferromagnetic material, thereby conferring characteristics of excellent linearity on the coil because of the absence of saturation.
A Rogowski coil is particularly suitable for use in measuring the magnitude of current at mains frequency.
To do this, the coil is loaded with a resistor R of high resistance.
The voltage picked up across the terminals of the coil is an image of the derivative of the current, providing that R.sup.2 is much greater than r.sup.2 +L.omega..sup.2 where r designates the resistance of the coil, L designates its self-induction, and .omega. is the angular frequency of the current to be measured.
An image of the primary current can then be obtained merely by integrating said voltage.
One of the difficulties encountered in measuring current by means of a conventional type of Rogowski coil results from the way the parameters of the coil vary with temperature, thereby giving rise to large errors of measurement.
Another difficulty comes from the fact that it is difficult to make a conventional type of coil that is perfectly symmetrical, axially; unfortunately, if the coil is not perfectly symmetrical, then the measurement depends on the position of the conductor passing through the torus and also on its orientation.
Axial symmetry requires, at least, that the winding be constituted by at least a double (go-and-return) winding which is technically difficult if not impossible to achieve when winding wire around a torus and which, in any event, ensures that a coil made in this way is very expensive.